Quantifying behavior is a fundamental problem in many academic disciplines and for a variety of theoretical problems ranging from behavioral ecology and ethology through developmental psychobiology and neurophysiology to veterinary medicine. Researchers often use computer programs called 'event recorders' to assist in the quantitative study of behavior. Existing programs only run on one type of computer (i.e., they only work in DOS, Windows or Macintosh operating systems), typically have constraints that interfere with understanding the structure of behavior, and may cost thousands of dollars per copy. We have developed a working prototype of an event recorder and data analysis program written in the Java TM computer language that operates on any modern microcomputer. We have included novel algorithms that permit users to score behavior once and analyze the resulting data record many times to obtain a deeper understanding of the structure of behavior. The prototype is designed to work in concert with existing spreadsheet and graphics programs. We aim to complete development by: updating it so that it can run on the Java operating system of the future (Java 2); adding the ability to combine results files and measure observer reliability; adding sequential analysis algorithms; adding graphical output so that users will not have to use other programs to visualize behavior; developing an integrated event-recorder for Palm operating system PDAs so that users will be able to score behavior under field conditions and analyze it using JWatcher. We distribute JWatcher without charge to the research community and intend to continue doing so. Our goal is to provide a powerful tool for research and teaching for the biomedical community that facilitates the study of behavior without spending scarce research funds on commercial software. While we will continue to freely-distribute the software, on-going support and routine updates will be supported by the sales of a detailed users manual, which will include laboratory exercises suitable for teaching.